1. Field of the Invention
The present invention relates to a production method for silicon wafers including forming vacancies in a silicon wafer by thermal annealing in atmosphere and forming a denuded zone (DZ) layer just under the surface of the silicon wafer by further thermal annealing, and to silicon wafers produced by this method.
2. Description of the Related Art
Silicon wafers produced by processing single crystal silicon which is obtained by pulling up and growing by Czochralski (CZ) method contain many oxygen impurities, and the oxygen impurities generate oxygen precipitates (BMD: bulk micro defect) causing dislocation, defects, or the like. When the oxygen precipitate is on the surface on which a device is to be formed, the oxygen precipitate causes increase in leakage current, decrease of dielectric breakdown voltage of oxide films, and the like, and as a result, characteristics of a semiconductor device are remarkably degraded.
Conventionally, thermal annealing (RTA: rapid thermal annealing) with rapid heating at high-temperature of 1250° C. or higher for a short time and rapid cooling is carried out on silicon wafers in the predetermined atmosphere to form vacancies with high concentration in thermal equilibrium in the silicon wafer. Subsequently, since the silicon wafer is frozen by rapid cooling while vacancies are out-diffused on the surface by thermal annealing, a DZ layer (a non-defect layer) is uniformly formed (for example, as disclosed in WO 98/38675). After forming the DZ layer, thermal annealing at lower temperatures than the above temperature, which allows formation and stabilization of oxygen precipitated nuclei, is carried out in order to form a BMD layer which has a gettering effect as a defect layer in the silicon wafer.
Furthermore, as another conventional technique (for example, disclosed in WO 98/45507), thermal annealing in an oxygen atmosphere is carried out on the silicon wafer, and subsequently, the thermal annealing in a non-oxidizing atmosphere is carried out, to form a DZ layer near the surface and a BMD in the silicon wafer.
In the thermal annealing for forming vacancies, conventionally, N2 (nitrogen) has been mainly used as the atmosphere. Vacancies are injected into the silicon wafer by decomposing N2 at high temperature and forming SixNy (nitride film) on the surface of the silicon wafer.
However, using the above-described thermal annealing for silicon wafers cannot solve the following problems.
Conventionally, the thermal annealing for forming vacancies is carried out in the atmosphere mainly containing N2 after covering the surface of the silicon wafer with an oxide film. To obtain sufficient thermal annealing effects, thermal annealing at 1250° C. or higher for 10 seconds or more is necessary. As a result, slips are generated from parts being in contact with a susceptor, supporting pins, or the like, by the thermal annealing at high temperature in the silicon wafer. These slips may cause cracks or the like.
Furthermore, a natural oxide film is formed on the surface of the silicon wafer before thermal annealing. The natural oxide film on the surface of the silicon wafer sublimates during the above thermal annealing at high temperature, so that the surface of the silicon wafer becomes rough.